Microphone unit



March 22, 1960 J. [RICK 2,929,877

MICROPHONE UNIT Filed Oct. 9. 1953 2 Sheets-Sheet 1 INVENTOR.

JOHN [RICK F|G.3. BY

March 22, 1960 J. IRICK 2,929,877

' I MICROPHONE UNIT Filed Oct. 9. 1953 2 Sheets-Sheet 2 INVENTOR. .JOHNIRICK /S ATTOP/VEVJ MICROPHONE UNIT John Irick, Huntington, N.Y.,assignor to Telephonics Corporation, Huntington, N.Y., a corporation ofNew 'ork Application QCiObeI 9, 1953, set-in No. sssnso 2 Claims. 0.179-4) carbon granules as the microphone is handled. In attempting toobtain a suitable microphone not susceptible to the disadvantages ofcarbon microphones, various expedients have been tried, including use ofmagnetic and dynamic types of microphones, but both types requireelectron tube amplifiers, since the output level of magnetic or dynamicmicrophones is relatively low in comparison with that or the carbonmicrophone. Inasmuch as the electron tube amplifiers required formagnetic or dynamic microphones materially increase the bulk of theequipment and susceptibility of damage, the magnetic or dynamicmicrophones, although free of the disadvantages of carbon microphones,introduced additional problems of equal or greater magnitude,particularly when the apparatus is intended for portable or fieldservice.

In accordance with the present invention a microphone unit is providedwhich is unaffected by humidity, does not change its characteristicafter long use, is relatively insensitive to shock, and is arranged tobe directly interchangeable with the conventional carbon type microphonein existing transmitters without adaptation change of the latter.

A preferred embodiment of the microphone unit of this inventioncomprises a magnetic or dynamic type microphone, a low noise unit ascompared with a carbon microphone, coupled to a transistor amplifier,all of these components being contained in a standard carbon microphonehousing. Consequently the low'noise microphone unit may be directlyplugged into a standard carbon microphone receptacle. The same lowvoltage supply adequate for operation of the carbon microphone issufficient for the operation of the new substitute microphone unitwhichalso utilizes the power feed lines for transmitting the audio signalstherefrom.

It will .be seen that the new microphone unit simultaneously overcomesall of the operational disadvantages of a carbon microphone and retainsthe advantages of compactness and sufiiciency of low input power,and'adds the valuable advantage of direct substitution for the "carbonmicrophone in existing equipment without any change therein.

For a more complete understanding of the invention reference may be hadto the accompanying drawings in which:

Figure 1 is a perspective view of a microphone unit embodying thepresent invention;

Figure 2 is a schematic circuit diagram of atypical i. tt

power supply that may receive a microphone unit constructed inaccordance with the present invention;

Figure 3 is a schematic circuit diagram of a transistor amplifier thatmay be employed in the present invention; Figure 4 is a schematicdiagram of another transistor amplifier that may be utilized in thisinvention, and

Figure 5 is a schematic diagram of a further transistor amplifier thatmay be employed in the invention.

Referring to the drawings and more particularly to Fig. 1, a microphoneunit in accordance with the invention is enclosed in the microphone case10 and includes a magnetic or dynamic microphone, which are inherentlylow noise units, mounted behind the perforated mouthpiece 11 as well asthe amplifier components described in case 10 and coupled to a standardmicrophone plug 14.

A microphone jack 14a, coupled to standard carbon microphone powersupply and amplifier circuits 15, receives the plug 14.

A representative type of standard carbon microphone power supply andamplifier circuits 15 which have been conventionally employed inassociation with carbon type microphones are shown in Fig. 2, it beingunderstood that other circuits performing similar functions may besubstituted therefor. A source of 3+ voltage is coupled to the jack 14aby a conductor 25a to dropping resistors 16 and 16a having suitablevalues and to a damping resistor 17' in turn joined to one side of aprimary winding 1% of an input transformer 1h.

The other side of primary winding 19a is joined to the junction of theresistors 16 and 16a and coupled through a relatively high capacitycondenser 18 to ground. The

secondary winding 1% of the transformer Hleads into a conventionalamplifier 15a as does a conductor Zea joined at its other extremity tothe jack 14a. A lead Z'Fa is com nected to a terminal on the jack 14aand to ground.

The components mounted in the microphone case 31% are diagrammaticallyshown in Fig. 3 asv including the magnetic microphone 26 directlycoupled to a base 22 thereon and having an output impedanceapproximately matching the input'impedance of a transistor 21. In thecircuit of Fig. 3 it is preferable to employ a junctiontransistor suchas the Raythcon CK721, for example. It will be understood that when analternative low noise microphone is employed in place of the magneticmicrophone 20, an input transformer will be employed to properly matchthe impedance, of such a microphone to the input impedance of thetransistor 21.

In order to supply the necessary voitage for proper operation of thetransistor amplifier, a conductor 25, adapted to be connected throughthe cable 13 and the plug 14 to the conductor 25a, is coupled throughthe switch 12 and a secondary winding 28 of an output transformer 29 toan emitter 24 on the transistor 21.

The positive side of a condenser 34 is connected to the 3+ terminal ofswitch 12 through the winding 28 and to ground through a bleeder networkconsisting of resistors 39 and 31. The resistors 3t and 3?. are sochosen that thevoltage drop across the resistor 3t} places the emitter2d of the transistor 21 at a slightly higher posisive potential than thebase 22, an arrangement necessary when the PNP junction type transistoris employed. Obviously, an NPN type transistor may be utilized withminor circuit changes. The value of the resistor 30 is chosen to limitthe current fiow to a collector 23 to a safe operatingrange for thetransistor-2i.

output transformer 29 in this circuit, the primary and seca substancesuch as a polyester resin in'the shape to fit in the case 10, withprojecting terminal plugs adapted to be inserted in the existing socketsin the case 10. The unit is accordingly compact and fits within the case19in the same space formerly occupiedby a carbon micro: phone unit withthe diaphragm of microphone 20 posie tioned behind'the mouthpiece 11.

in a typical operation of this embodiment of the invention, the plug 14of the microphone unit in the case 10 is inserted in the jack 14a of thepower and amplifier circuits 15. The actuation of the button 12a closesthe switch 12 and results in the energization of the transistoramplifier as well as the amplifier 1.5, due to its being groundedthrough the conductor 26. Upon the impingement of sound waves on themagnetic microphone 20, a signal will be introduced to the base 22causing .the collector current flowing through the primary winding 32and the collector 23 to vary, such variation-being transmitted to thesecondary winding 23.

Since the condenser 34 has a relatively high capacity, one side of thesecondary winding 28 is at audio ground and the output signals will betransmitted to. thejack 14 through the conductors and 27. The relativelyhigh capacity of the by-pass condenser 34minimizes feedback of the audiosignals; From the plug 14', 'the audio signals will be transmittedthrough thejack 14a, the conductor 25a, the resistor 17, the transformerwinding 19a and the condenser 15 to ground. Since the condenser 18 has alarge capacity, it will by-pass the audio signals therethrough'so thatthe primary winding of the transformer 1!) is effectively tied to theground for audio signals. The audio signals will then be coupled intothe amplifier 15a and subsequently to any desired radio or telephonetransmitting circuit.

In an exemplary embodiment of this form of the inven tion, the circuitcomponents may take the following values; transistor 2l=RaytheonCK-72l,'resistor 30:4.5 ohms, resistor 31:220 ohms, condenser 34:20mfds, magnetic microphone 20:700 ohms output impedance, loadimpedance=90 ohms, B+ voltage (at condenser 34)=9 v.

Employing these circuit constants, the unit shown in Fig. 2 will deliverbetween 36 db mv. and 44 db mv. of output with 115 db sound power levelat .the microphone. The noise level was 16 db mv. which is 16 db lowerthan the equivalent carbon microphone. However, there is 6 db less gainin this circuit than in the carbon microphone so that an actual 10 dbsignal to noise gain is obtained.

Referring next to Fig. 4, a circuit arrangement employgroundbetwe'en thetransistors 21and 41, if found necessary to limit the collector currentof the transistor 41. However, it has been foundthat the system willoperate in many instances without such a resistor.

The collector circuit of the transistor 41 includes a primary winding 47of an output transformer 46, similar to the transformer 29 found in Fig.3, and having a secondary winding 48, one side of which is connectedthrough the switch 12 to. the conductor .25 and the other side connectedto the primary winding 47 and to the coudenser 3 4. Since'the transistor41 is of the NPN type, the collector 43 is biased positively withrespect to the emitter 43 and the 'base 42 by-the B+ supply. through thesecondary winding 48 and .the primary winding 47.

The operationof the embodiment of the invention illustrated in Fig. 4 issimilar to that described in connection with Fig. 3, with the exceptionthat the signal appearing at thecoll'ector 23 of the transistor 21 isfurther amplified by the transistor 41 and coupled therefrom to theoutput transformer 46, and subsequently to the power and amplifiercircuits 15. In comparing the performance of the circuit of Fig. 4'withthat of Fig. 3, the additional transistor stage-introduces slightly morenoise into the output so that the noise under quiescent conditions isapproximately the same as when a carbon microphone is employed. However,due to a gain increase of about 6 db over the carbon microphone, anoverall improvement of about 6 db in the signal to noise ratio isprovided by this arrangement. Furthermore, of course, the advantages ofimmunity to severe environmental conditions outlined above are. obtainedby employing this circuit.

Reterring next to Figure 5, another circuit thatv may be employed in thepresentinvention is shown. The chief difference between the circuit ofFig. 5 and the previously described circuits resides in the employmentof a cascade arrangement of transistors that eliminates the somewhatbulky by-pass condenser. Elements. in this circuit similar to thosepreviously described have been designed by the like referencecharacters. Describing this circuit in more detail, the magneticmicrophone 20 is coupled at one end to the base of a junction transistor50 of the PNP type and at the other end to the base of asimilartransistor 51. The proper bias voltages are atforded thetransistors Sit and 51 by a bleeder network including the resistors 30and 31 connected to the 13+ voltage supply at the conductor 25 through asecondary winding 52 of an output transformer 54 and the switch =12. Thecollectors of the transistors 50 and 51 are joined to opposite sides ofa primary winding 54 of the output transformer 53. i In a typicaloperation of the circuit shown in Fig. 5, .the,signals derived from themicrophone 20 are ampli- .fied by the transistor 50 and applied to thesecondary ing many of the components found in Fig. 3 is shown, so

from the collector 23 to a second junction transistor 41 of the NPN typehaving a base 42,'a collector 43 and an emitter 44. A resistor 45 may beconnected to winding 52 from the primary winding 54. A portion of theseoutp it signals will be fed back to the base of the transistor 51 acrossthe resistor 30 in phase with the signals from the microphone 20. Theresult .will be an increase in the signal strength in the winding 52. gIt will be undersood that the above described embodiment of theinvention is illustrative only and modifications thereof will occur tothose skilled in the art. Thus, point type transistors could besubstituted for the junction type transistors described in connectionwith the present arrangements although at the present time, the junctiontype are preferable due to the lower noise level and higher gain thatmay be obtained therewith. Furthermore other known transistor amplifiersmay be incorporated in the invention. Also, the amplifier circuit may bemolded into the cord 13 near or at the plug 14 when the microphone case10 must be of a minimum size to be as inconspicuous as possible.Therefore, the invention is not to be limited to the specific apparatusdisclosed herein but is to be defined by the appended claims.

Iclaim:

l. A'microphone unit for use with a'staudard carbon microphone powersupply and amplifier circuits to replace a high noise level carbonmicrophone comprising a housing, a low noise microphone mounted in saidhousing, a transistor amplifier mounted in said housing and connected tosaid microphone, a cable including a plurality of conductors connectedto said transistor amplifier, said cable extending ing power to andtransmitting signals from said transistor amplifier on the sameconductors, and a plug connected to the end of said cable outside ofsaid housing adapted to be plugged into the standard carbon microphonepower supply and amplifier circuits.

2. A microphone unit as defined in claim 1, wherein a switch includingan actuating arm is located in the from said housing for supplyhousingand connected in at least one of the conductors leading from thetransistor amplifier, said arm extending through said housing tofacilitate actuation of the switch.

References Cited in the file of this patent UNITED STATES PATENTS2,380,438 Hornickel July 31, 1945 2,406,583 Blauvelt Aug. 27, 19462,579,162 Veneklasen Dec. 18, 1951 2,613,282 Scaife -Oct. 7, 1952FOREIGN PATENTS 860,641 Germany Dec. 22, 1952

